Generally, during pregnancy, the fetal health condition is monitored by assessing fetal heart rate variability and fetal movements. The degree of fetal heart rate variability provides indirect information on the physiological state of the fetus, e.g. when the fetus is asleep the fetal heart rate variability is expected to be smaller than when the fetus is awake and active. The frequency with which transitions between these physiological states occur is used by physicians to monitor the development of the fetus with progressing pregnancy. As stated, the fetal heart rate variability is an indirect parameter to assess the physiological state of the fetus. Another parameter from which the state can be determined is the degree of fetal movement. However, occurrence of fetal movement can only be established through subjective assessment of the mother or by ultrasonic echo recordings.
During pregnancy, Doppler ultrasound is the most widely used method to monitor the fetal heart rate. However, due to the small size of the fetal heart and vascular system the resolution of the Doppler ultrasound signal is small. Moreover, the ultrasound probe requires frequent repositioning as a result of movement by either the mother or fetus. Another disadvantage of this method is that, when it is combined with ultrasonic fetal movement monitoring, two ultrasound probes are required. Not only is this more demanding for physicians, but also do both these probes feed energy to the fetal body, which potentially affects fetal health.
In “Limitations of autocorrelation in fetal heart rate monitoring” by Fukushima, T. et al., in Am. J. Obstet. Gynecol. 1985; 153:685-692, a fetal heart rate monitor is described that produces a fetal heart rate. Also, autocorrelated ultrasound fetal heart rate records are compared to simultaneously recorded direct scalp fetal electrocardiogram tracings, and the potential misinterpretation of autocorrelated fetal heart rate data is discussed.